1. Field of the Invention
The present invention is directed to a protective tube arrangement for a piston-cylinder unit having a piston rod.
2. Background of the Invention
A piston-cylinder unit having a protective tube arrangement of the generic type is sufficiently well known and is used, for example, as a suspension strut in the automotive industry.
Generally, the piston-cylinder unit comprises a working cylinder which is filled with a working medium and a piston rod which is fixed at least indirectly to a piston and which is axially guided in the working cylinder. To prevent the escape of working medium out of the interior of the cylinder, the piston rod is sealed radially relative to the working cylinder by means of a piston rod seal; this piston rod seal radially encloses the piston rod and presses against the piston rod, and the outer circumferential surface of the piston rod seal presses against the inner wall of the working cylinder. The piston rod which is fixed to the piston moves into or out of the working cylinder depending upon the direction of the axial piston movements. To reduce the friction occurring between the piston rod and the piston rod seal, the surface of the piston rod is optimized to achieve a defined surface finish quality. It is important to protect the piston rod against possible damage caused by shocks or soiling because otherwise the piston rod seal is irreparably damaged by scratches and contamination of the piston rod surface.
A protective tube which is at least indirectly fixed to the exit-side end of the piston rod and which at least partially encloses the piston rod so as to form a radial clearance is commonly used to protect the piston rod. In doing so, the protective tube covers the portion of the piston rod extending out of the working cylinder in every state of the suspension.
A protective tube made of steel is commonly pressed on or welded to the carrying cap which is fixed to the piston rod.
It is likewise known to use a tube made of plastic. The low weight of a protective tube made of plastic is considered particularly advantageous. Apart from this, a plastic protective tube is highly resistant to corrosion so that additional varnishing of the protective tube can be dispensed with.
DE 1 951 753 U discloses a protective tube made of plastic in which the inner surface thereof has an inclined surface which narrows conically in direction of the cover plate and which terminates by an annular groove at the end of the protective tube. The connection of the protective tube to the carrying cap secured to the piston rod is carried out by pulling the protective tube over the carrying cap which accordingly slides along the inclined surface and locks into the annular groove.
However, the special shape of the protective tube cannot be produced without substantial expenditure. Further, the production of the protective tube requires a high precision because if the annular groove is made too deep and/or too wide the connection will exhibit play between the protective tube and carrying cap. If underdimensioned, the carrying cap made of metal can destroy the protective tube made of plastic when pulled on.
DE 1 995 281 U discloses a protective tube which is made of plastic and which has at least one thermally formed annular constriction by which it is fastened to the carrying cap. The constriction can be generated during the assembly process, for example, by means of thermoelectric pincers.
However, this method can only be realized in case of protective tubes which are produced from thermoplastics, which severely limits the range of possible materials for producing a protective tube.
It is an object of the present invention to develop a fastening of the protective tube to the carrying cap which can be produced in the simplest and most economical way possible and which is suitable for protective tubes produced from thermoplastics and duroplasts or metal and which solves the problems mentioned above.